About electric double layer capacitors, wherein an electric double layer made of a polarized electrode and an electrolytic interface is used, in particular, coin-shaped ones, the demand thereof as memory backup power sources has been rapidly expanding in recent years. Meanwhile, attention has been paid to the use of electric double layer capacitors, which are characterized by having a high power density, for articles for which a large capacity is necessary, such as power sources for electromobile.
As the electrode material of electric double layer capacitors, a carbonaceous material, such as active carbon, is mainly used. In order to cause the carbonaceous material to be held on a current collector, the carbonaceous material is used by mixing with a binder polymer, which may be referred to merely as a binder hereinafter. As a conventional binder for an electrode for an electric double layer capacitor, polytetrafluoroethylene (PTFE) has been used. As the method for binding a carbonaceous material containing a binder onto the surface of a current collector, there has been adopted a method of incorporating and dispersing the carbonaceous material into a solution or latex of the binder to prepare a slurry (in the form of a homogeneous paint), applying the slurry onto the current collector, and then drying the applied slurry.
However, in the case of using PTFE as the binder, the following problems are caused: when the amount thereof is small, the binding force thereof to the current collector is insufficient; and when the amount is large, the internal resistance of the electrode becomes high.
Against the above-mentioned problems, there have made some approaches for supplying an electrode which has flexibility and excellent binding force by use of a small amount of an elastomer as the binder. For example, suggested are a method of using a styrene-butadiene based polymer having a specific composition as the binder (see Japanese Patent Application Laid-Open (JP-A) No. 11-162794); and a method of using a mixture of a styrene-butadiene based polymer and a cellulose based polymer as the binder (see JP-A Nos. 2000-208368 and 2001-307965).
However, these binders have a problem of being poor in heat resistance. In other words, the binders have the following problem. Active carbon, which is used as electrode material, easily adsorbs water since active carbon has a large specific surface area. It is therefore necessary to dry the slurry thereof applied onto a current collector at high temperature so as to remove water therefrom. However, when a styrene-butadiene based polymer is used as the binder therefor, its flexibility is lost by the high-temperature drying so that the active carbon is peeled from the current collector or the electrochemical stability lowers.
Thus, an object of the present invention is to provide a new binder composition for electric double layer capacitor which comprises a binder having a good binding force and an excellent heat resistance, and an electrode having the binder. Another object of the invention is to provide an electrochemically stable electric double layer capacitor having a high capacity.